1. Field of the Invention
This invention relates generally to pressure transducers and measuring devices and more particularly to differential pressure transducers and measuring devices employing piezoresistive strain sensitive elements as sensors. Piezoresistive elements are elements whose resistance changes proportionately to an induced strain.
2. Description of the Prior Art
The prior art has shown the use of piezoresistive strain sensitive elements in the measurement of force. In many examples of the prior art, these piezoresistive elements are bonded or otherwise attached to a flexible diaphragm and connected together to form a Wheatstone bridge. Fluid pressures in particular have been commonly measured using edge supported flexible diaphragms that are arranged to deflect concavely or convexly in response to pressure differentials on opposite faces of the diaphragm. As the diaphragm deflects in response to an applied pressure, tension and compression forces are created that may be used to measure the applied pressure as a function of the deflection of the diaphragm. That is to say, the resulting strain created in the diaphragm is a function of the stress causing that strain. In turn, the deflection of the diaphragm may be determined as a function of the change in resistance of suitably mounted piezoresistive strain sensitive elements. The prior art is replete with arrangements of these strain sensitive elements so as to give optimum signal output as a function of position on a diaphragm. Another prior art development has been concerned with the employment of diffusion techniques in the semiconductor art to form Wheatstone bridge connections between compressive and tension gages formed on a single semiconductor chip. Some of these prior art devices have been brought to the attention of the applicant's attorney through a search conducted at the United States Patent and Trademark Office and are listed as follows:
______________________________________ PATENT NO. INVENTOR ______________________________________ 3,247,719 Chelner 3,624,714 Frassrand 3,744,317 Hessinger 3,149,488 Castro 3,230,763 Frantyis 3,239,696 Burkhalter 3,372,577 Bates 3,473,375 Jenkins 3,527,099 Herceg 3,537,319 Yerman 3,641,812 Vick 3,697,917 Orth 3,697,918 Orth 3,820,401 Lewis ______________________________________
The class and subclasses searched were:
______________________________________ CLASS SUBCLASS ______________________________________ 73 388 73 398 73 406 73 408 ______________________________________
While presently available pressure transducers can measure differential pressures to a high degree of precision, bonding methods are such that many of the piezoresistive elements can slip from their positions through failure of the bonding cement after repeated flexing and thus no longer occupy the optimum positions determined in their design and fabrication. It has also been inconvenient to protect the sensing elements from the pressurized media in some applications.
It would thus be a great advantage to the art to provide a piezoresistive type differential pressure transducer in which positional integrity of sensing elements is of a high degree.
It would be a further advantage to provide such a pressure transducer in which the sensing elements are isolated from the pressure supporting media.
It would also, of course, be a desirable advantage to provide such a transducer in a comparatively economical structure.